Add the mock factory of msg stream

[SimFG]

/kind improvement

[czs007]

/approve
/lgtm

[sre-ci-robot]

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[codecov]

Codecov Report

Merging #27834 (04a6efa) into master (babf4e3) will increase coverage by 0.01%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #27834      +/-   ##
==========================================
+ Coverage   81.71%   81.73%   +0.01%     
==========================================
  Files         819      819              
  Lines      116062   116062              
==========================================
+ Hits        94845    94867      +22     
+ Misses      18048    18027      -21     
+ Partials     3169     3168       -1     

see 12 files with indirect coverage changes